Allow for repo group publishing in Pulp
Pulp allows for repo groups to be published, but this functionality is currently not exposed in Runcible.
This feature is to add this to API passthrough to Runcible so Katello can publish repo groups. This consists of two parts:
- allow distributors to be added to repo groups (for now, only the export distributor)
- allow runcible users to call the publish API on a repo group
Additionally, the export distributor has been added as a valid option as yum repos (see https://pulp.plan.io/issues/1394). This is a small change that is related to repo group publishing, so I would like to include it in this ticket.